1. Field of the Invention
The present invention relates lasers. More specifically, the present invention relates to a system and method for cooling a high-power solid state laser.
2. Description of the Related Art
Solid state laser technology has advanced to the point where thermal management of high-energy lasers in operation is a limiting factor in scaling such systems to greater power levels. This is particularly true where high-energy laser system design parameters are constrained to limited size and weight. Thermal management technology advancement is therefore a key factor in increasing power output levels in high-energy solid state lasers.
Current solid-state laser designs employ a doped-insulator lasing medium driven by high power light emitting diode array pumplights. The lasing material is typically comprised of a host crystal doped with an ion, such as for example, ytterbium doped yttrium aluminum garnet (Yb:YAG). Rod shaped lasing media have been used, but are power limited due to the limited surface area they present for coupling pumplight into the medium and for removal of waste heat energy. High aspect ratio slabs are now employed to overcome some of the limitations present in rod type lasers. Lasing slabs are formed in high aspect ratio configurations that define two ends, two long and narrow edges, and two broad side surfaces.
Modern slab lasers are optically pumped by narrow spectral band, high brightness laser diode arrays. The higher brightness levels of such laser diode pump sources allows the high aspect ratio slab to be pumped either through the narrow edges of the slab, in directions generally transverse to the laser beam, or, through the narrow ends of the slab, in directions generally co-linear with the laser beam. Edge and end pumping of the slab allows the broad side faces to be cooled without constraining the cooling system to also transmit the pumplight beam into the slab, thereby generally simplifying the cooling system design by not requiring the coolant to transmit the pumplight beam. Laser efficiency is also improved with a pumping configuration that results in the optimum absorption and distribution of pump energy in the lasing medium.
In operation, pumplight energy is coupled into the laser slab and serves to excite ions in the lasing medium, which change energy states to produce the laser beam energy. Like all energy conversion processes, the efficiency of the lasing process can not achieve 100%. Energy that is not converted into laser beam energy is waste energy that results in the production of sensible heat energy and fluorescence light energy. The sensible heat energy must be conducted to the surfaces of the slab for removal while the fluorescence energy may be transmitted through the slab surfaces as radiated light energy. Both forms of waste energy must be removed from the system. Failure to remove the waste energy produces several deleterious effects that ultimately limit the maximum laser beam quality and the energy capability of the system.
Generally, an increase in operating temperature within the lasing medium reduces the efficiency of the lasing process. Conversely, reducing the operating temperature of the laser increases the gain and extraction efficiency. More specifically, reducing the operating temperature increases the stimulated emission cross-section of the active lasing medium. Similarly, this also lowers the saturation intensity, which makes it easier to extract power from continuous and high pulse rate systems without damage to the optical components in the system. Thus it is clear that high-energy laser systems benefit from effective thermal management. There have been various approaches to waste energy removal in the prior art.
Generally, heat and energy removal implies a flow of energy from within the lasing medium slab outward. The flow of sensible energy creates a temperature gradient within the slab. The temperature gradient causes mechanical stress within the slab. When the medium is stressed the crystal becomes birefringent. Birefringence causes energy in the laser beam, if polarized in a direction that is neither along nor orthogonal to the stress gradient, to become depolarized from the desired beam polarization. Such induced birefringence is therefore undesirable, particularly in high-energy applications. A typical multipass master oscillator power amplifier laser system uses a polarizer and 90 degree polarization rotation device to separate the master oscillator input beam from the amplified output beam. If beam polarization is compromised, because of thermal stress induced birefringence, a portion of the output beam is fed back into the master oscillator. Such feedback is potentially damaging to the oscillator components. Depolarization also reduces the output power and imprints a non-uniform intensity profile on the output beam, which adversely affects beam quality. It is therefore desirable to maintain a one-dimensional temperature gradient within the slab and orient the polarization of the beam to be co-linear with or orthogonal to this gradient in order to avoid depolarization due to thermal stress birefringence.
In side-pumped slab laser configurations, heat is removed from the lasing medium by cooling mechanisms applied to the broad side faces of the slab. Prior art methods for cooling the broad slab faces include air cooling, liquid cooling systems, and conductive cooling through metal heat sinks. Similar approaches have been applied to cool high power Yb:YAG laser rods, employing small jets that impinge liquid coolant directly on the surface of the rod, thereby improving the heat transfer properties. See for example; Phillips, et al, U.S. Pat. No. 5,471,491 and Bruesselbach, et al, U.S. Pat. No. 5,636,239. Cooling hot slabs of glass with multi-jet impingement is known in area of art concerning tempered glass. Multi-jet impingement cooling creates a variety of coolant flow issues that are not easily resolved when taken in combination with the need to control the thermal gradients in the slab. Further, the prior art teachings do not address the issue of the removal of the fluorescence energy required in application with solid state diode-pumped laser.
Thus, there is a need in the art for a system and method to remove waste energy from slab lasers that avoids the disadvantages of the prior art, while maintaining compact size, high efficiency, and low distortion of the laser beam.